The present invention relates to a rising pipe of sintered ceramic material for light metal melts. Such rising pipes are used in low-pressure casting for light metal melts, in particular aluminium and aluminium alloys, e.g. to DIN 1725, sheet 2, and related special alloys. In these processes the light metal melt is located in a sealed, thermally insulated container out of which the melt is fed into the metallic mold (die) via a rising pipe. After cooling of the mold and solidification of the melt in the mold the gas space above the melt is relieved of pressure, so that the melt contained in the rising pipe flows back into the container. In the gas space above the melt there are further located heating rods, which maintain the melt temperature in the container. In general heating rods of silicon carbide are used.
Whereas previously rising pipes of cast iron protected at the surface by a ceramic coating were used, the latter have been replaced in recent times by rising pipes of ceramic materials such as silicon nitride or aluminium titanate. Problems which arise through mechanical stresses in the flange areas by virtue of the brittleness of the ceramic material can be largely prevented by suitable shaping depending upon the particular material used. It is found, however, that the life of ceramic rising pipes has been limited due to the formation of certain types of cracks. These cracks are not caused purely by mechanical loading in the flange area or purely by shock or impact. Detailed investigations of these types of crack, which occur in particular in the upper third of the rising pipe above the melt level, have shown that they are caused by a combination of thermomechanical fatigue and chemical attack. The thermomechanical fatigue results from extreme and sharp temperature variations, which are caused by the thermal radiation arising from the heating (temperature above the melt temperature), the filling of the rising pipe with the melt and the loading of the gas space above the melt with relatively cool pressure gas. Air is frequently used as pressure gas, for reasons of economy, particularly in the case of magnesium-free melts. Moreover air is sucked in during the dropping of the melt from the casting mouthpiece level of the mold onto the melt level. The interaction of metal vapour with oxygen above the melt level, even when nitrogen is used as pressure gas, and only minor amounts of oxygen are present, leads to a chemical attack on the rising pipe.